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The Keck Interferometer is a two-telescope astronomical interferometer. It forms part of NASA's overall effort to find planets and ultimately life beyond our solar system. It combines the light from the twin Keck telescopes to measure the emission from dust orbiting nearby stars, directly detect the hottest gas giant planets, image disks around young stars and other objects of astrophysical interest, and survey hundreds of stars for the presence of planets the size of Uranus or it can be larger.

The Keck Interferometer is a ground-based component of NASA's Origins Program. Origins addresses fundamental questions about the formation of galaxies, stars, and planetary systems, the prevalence of planetary systems around other stars, and the formation of life on Earth.

At the Mauna Kea Observatory 4,150 meters (13,600 ft) above the Pacific Ocean, atop the dormant volcano on the "Big Island" of Hawaii, the twin Keck telescopes are two of the world's largest telescopes for optical and near-infrared astronomy. The Keck Interferometer joins these giant telescopes to form a powerful astronomical instrument. Each telescope stands eight stories tall and weighs 300 tons, and its primary mirror is ten meters in diameter. The mirror is composed of 36 hexagonal segments that may work in concert as a single piece of reflective glass.

On the 85 metre Keck-Keck baseline, the Keck Interferometer will have a spatial resolution of 5 milliarcseconds (mas) at 2.2 micrometres (µm), and 24 mas at 10 µm. In its most sensitive configuration, the interferometer would reach K=21 and N=10 mag in 1000 seconds of integration (SNR = 10 per baseline). The interferometer has several back-end instruments, allowing for a variety of observation types. The lack of additional outrigger telescopes makes the Keck Interferometer unsuitable for interferometric imaging, so work has concentrated on nulling interferometry and angular diameter measurements instead. In September 2005 the Keck Interferometer demonstrated nulling interferometry for the first time, with a modest null depth of 100 times.

The summit site, where the Keck telescopes are located, are surrounded by thousands of miles of relatively thermally-stable ocean, the 13,800 foot (4,206 m) Mauna Kea summit has no nearby mountain ranges to roil the upper atmosphere or throw light reflecting dust into the air. The area has few city lights and therefore less light pollution than urban areas. For most of the year, the atmosphere above Mauna Kea is clear, calm and dry.

Made possible through grants totaling nearly $200 million from the W. M. Keck Foundation and NASA, the observatory is operated by CARA for the California Institute of Technology, and the University of California, with the National Aeronautics and Space Administration (NASA), which joined the partnership in October, 1996. The Keck I telescope began science observations in May 1993; Keck II began operation in October 1996. The Keck I telescope is also used independently for Aperture Masking Interferometry measurements.

• List of astronomical interferometers at visible and infrared wavelengths

• NASA JPL's Planetquest page on the Keck